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Comet-invader Philae goes silent, mothership Rosetta forced to retreat

It's almost as if the strange rock doesn't want to be conquered by humans

By Iain Thomson, 20 Jul 2015

The European Space Agency (ESA) has all but lost contact with its Philae lander on Comet 67P – and attempts to reestablish communications are being hampered by the rock itself.

Philae, which was dropped off onto the comet from its mothership Rosetta, has more or less been silent for 11 days.

Rosetta, which still orbits Comet 67P, has been flying as low as 153km above the rock's surface as the spacecraft searched for a way to contact its Philae lander. But in the last week, the dust coming off the comet has obscured Rosetta's star trackers, which are used by the ESA team to navigate safely. As a result, the mothership has been shifted up into a higher orbit of between 170 and 190km from the surface.

"Although the mission will now focus its scientific priority on the orbiter, Rosetta will continue attempting – up to and past perihelion – to obtain Philae science packets once a stable link has been acquired," said Patrick Martin, Rosetta mission manager.

Earlier this month, the Philae lander had been able to communicate with Rosetta, but connections have been sporadic and incomplete. The lander, which bounced across the comet after its harpoons failed to get a solid hold on the surface, had signaled that it had enough power for operations a couple of weeks ago, but ESA now thinks something else could have gone wrong.

Philae uses two transmitting and receiving units for communications. But the team now thinks that the probe has one damaged upload and one broken download unit. It's also possible that the lander has shifted slightly as the comet melts and this may have moved its antenna to a new position.

"Philae is obviously still functional, because it sends us data, even if it does so at irregular intervals and at surprising times," said Philae lander manager Stephan Ulamec. "Several times we were afraid that the lander would remain off – but it has repeatedly taught us otherwise."

With communications so irregular, the ESA team is trying to send a "safe block" set of activities for the lander to carry out, such as taking temperature measurements, gas analysis for organic or inorganic substances, and acoustic tests on the surface and below.

In the meantime, Rosetta will move into its new orbit and examine the comet as it approaches its closest point to the Sun next month, especially the southern portion of the comet that is just now coming into full sunlight.

"Studying these regions is an important part of our long-term science goals in the lead-up to and beyond perihelion next month, when the comet is at its closest to the Sun along its orbit and activity will be at a maximum," said Nicolas Altobelli, acting Rosetta project scientist.

In the meantime, the probe will carry on sporadic attempts to contact the lander and get it producing science again. But it's looking increasingly likely that the next readings we get from the comet's surface will be when Rosetta attempts its own landing late next year. ®

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